Working with the BRCs
- 1 Downloading the data
- 2 Making the mosaics
- 3 Getting data from other wavelengths
- 4 Investigating the mosaics
- 5 Previously identified sources
- 6 Doing photometry
- 7 Bandmerging the photometry
- 8 Working with the data tables
- 9 Making color-color and color-magnitude plots
- 10 Making SEDs
- 11 Analyzing SEDs
- 12 Writing it up!
Downloading the data
Using the SHA, a concrete example was tuned to last year's project, and is already slightly out of date. I need to update it, and do something customized for you. Let's work through this one together, as a big group.
Big picture goal: Get you comfortable enough to search for your own favorite target, understand what to do with the search results, and download data.
More specific shorter term goals: Search on our targets. Understand the difference between the observations. Understand why I chose to use the observations that I did.
Relevant links from L1688 page or elsewhere: Using the SHA, a concrete example
Questions for you:
- Compare the various AORs you get as your search results when you search by position. How are they the same/different? Which do we want to download?
Making the mosaics
In the generic case for most targets, you can probably use the online mosaics that come as PBCD (Level 2) mosaics (or delivered products, if they exist for the region you want -- see "inventory search" in the SHA). In this case, we can use the online mosaics.
Big picture goal: Recognize at a glance what is an instrumental artifact and what is real.
More specific shorter term goals: Look at the online mosaics. Understand what is part of the sky and what is not. Understand which I reprocessed and why.
Questions for you:
- Compare the mosaics across the bands, online and mine. What changes? What stays the same? Why?
- What is saturated?
- Notice the pixel scale. What is the real pixel scale of IRAC (and MIPS)? What are the pixel scales of the images? Does that actually change the resolution? (for advanced folks - why did I do this?)
Getting data from other wavelengths
You have already made some progress on this in your literature search this Spring, but there are a TON more data we can mine.
Big picture goal: Understand how to use the various archives to find non-Spitzer data.
More specific shorter term goals: Go get data for both BRCs for comparison to our Spitzer data, both images and catalogs.
Relevant links from L1688 page or elsewhere: How can I get data from other wavelengths to compare with infrared data from Spitzer? and Resolution
Questions for you:
- Figure out how to get data from Akari, WISE, 2MASS, MSX, IRAS, IPHAS, POSS, SDSS (may not have any hits), and anyplace else you want. Which will give you images, and which will give you catalogs (not all will give you both)? Go do it. For images, if you are using Skyview from Goddard, make sure to worry about pixel scale. Best to try to go direct to the source for these archives, rather that relying on Goddard. Get images covering about the same area as the Spitzer images so that they are easy to compare, but larger scale images might be useful to give a sense of context too.
- For each catalog: What wavelength is this? How is it relevant to YSOs? How is the resolution different? (You may need to do the next section before you can answer this.)
Investigating the mosaics
It is "real astronomy" to spend a lot of time staring at the mosaics and understanding what you are looking at. Don't dismiss this step as not "real astronomy" just because you are not making quantitative measurements. This is time well-spent.
Big picture goal: Understand what is seen at each Spitzer band and all the other archival bands.
More specific shorter term goals: Recognize how the images differ between the two BRCs, and among the various bands.
Relevant links from L1688 page or elsewhere: How can I make a color composite image using Spitzer and/or other data? and the questions on that page.
Questions for you, among just the Spitzer images:
- How does the number of stars differ across the bands? Which band has the most stars? The fewest? (Bonus question: why?) The most nebulosity? The least? (Bonus question: why?) Are there more stars in the regions of nebulosity, or less? Why?
- What other features are the same across the bands?
- Do the star counts differ between the two BRCs? Why?
- Which objects are saturated, in which bands?
- How big are any of the features in the image (nebulosity, galaxy, space between objects)? (What do I mean by big?) in pixels, arcseconds, parsecs, and/or light years? (Hint: you need to know how far away the thing is. If it helps, there are 3.26 light years in a parsec.)
- Make a three-color image. What happens when you include a MIPS-24 mosaic in as one of the three colors with IRAC as the other two? Do the stars match up? Does the resolution matter? Can you tell from just a glance at the three-color mosaic which stars are bright at MIPS wavelengths?
Questions for you, among all bands you can find:
- Figure out how to get data from Akari, WISE, 2MASS, MSX, IRAS, IPHAS, POSS, SDSS (may not have any hits), and anyplace else you want. (See prior task too.) Line them up with the Spitzer images of comparable wavelengths (e.g., 8 um with 12 um, 25 um with 24 um). How much more detail do you see with Spitzer that was missed by IRAS or the other missions? Do you see more texture in the nebulosity? More point sources? How does the resolution and sensitivity vary?
- Which features are found across multiple wavelengths? Why?
Previously identified sources
You've already done this as part of our proposal.
Big picture goal: Understand what has already been studied and what hasn't in the image.
More specific shorter term goals: Determine if the previously-known objects are saturated or not.
Questions for you:
- For each of the known objects, you have the RA/Dec - what are the pixel coordinates in the image? Does it change among the IRAC bands? In the MIPS band?
OK, this step is doing to take the longest, be the most complex, involve the most steps and the most math.
Never just trust that the computer has done it right. It probably did what you asked it to do correctly, but you asked it to do the wrong thing. Always make some plots to test and see if the photometry seems correct.
Big picture goal: Understand what photometry is, and what the steps are to accomplish it. Understand the units of Spitzer images. Understand how to assess if your photometry makes sense.
More specific shorter term goals: Do photometry on a set of mosaics for the same sources. Assess whether your photometry seems right.
Relevant links from L1688 page or elsewhere: Units and Photometry and I'm ready to go on to a more advanced discussion of photometry and Aperture photometry using APT and CG4 photometry: more detailed instructions
Questions for you:
- Use APT to explore the various parameters. What is a curve of growth?
- What are the best parameters to use? (RTFM to find what the instrument teams recommend.) What are the implications of those choices? What happens if you use other choices?
- We should decide as a group which set of sources to measure, and have everyone measure the same sources. We will then compare all of our measurements among the whole group.
- Compare the MOPEX source identifications I did from just one band with their corresponding image. Is it getting fooled by detector artifacts? you will need skyview for this, because you have the tbl files, as opposed to region files. Let me know if you want the reg files and I'll make you some.
- Compare the MOPEX source identifications from, say, IRAC band 3 with the image from IRAC band 1, or the source extractions from MIPS-24 with image from IRAC band 1. Are there a lot of stars (or other objects) in common? How does the nebulosity affect it? you will need skyview for this, because you have the tbl files, as opposed to region files. Let me know if you want the reg files and I'll make you some.
- Why did this happen? (see below)
Bandmerging the photometry
I use my own code to do this; there is no pre-existing package to do this. If you do it by hand (or semi-by-hand) using APT, you can manually merge the photometry. My merged photometry includes J through M24.
Big picture goal: Understand what this process is.
More specific shorter term goals: Do this by hand.
Relevant links from L1688 page: Resolution
Questions for you:
- Make sure that I've merged the right sources across several bands by spotchecking a few of them. (Find an object that the catalog says is detected in at least 3 bands and then overlay the images in a 3-color image or Spot to see if there is really a source there, at exactly that spot, in all bands, or if it's a cluster of objects, or different objects getting bright at different bands.
- Have I 'lost' the instrumental artifacts you noticed in the previous section? Or are there instrumental artifacts or otherwise false sources sill in the list?
- Does resolution matter? (Can you find a place where more than one IRAC source can be matched to the same MIPS source?)
Working with the data tables
Big picture goal: Understand how to work with the tables. Understand how to convert magnitudes back and forth to flux densities.
More specific shorter term goals: Import the table into excel. Program a spreadsheet to convert between mags and flux densities.
Questions for you:
- How many stars are detected in each band? Is this about what you expected based on your answer to the questions in the mosaic section above? HINT: you can do this using Excel, you don't need to count these manually!! Ask if you need a further hint on exactly how to do this.
- Which stars in the catalog are the stars identified in the literature?
- Make sure you understand how I got the magnitudes from the fluxes (or the fluxes from the magnitudes). You will need magnitudes for the next step, and fluxes for the step after that.
Making color-color and color-magnitude plots
Big picture goal: Understand what plots to make. Understand the basic idea of using them to pick out certain objects.
More specific shorter term goals: Make some plots. Understand the basic approach of Gutermuth et al. (see Gutermuth et al. 2009, Appendix A -- this was handed out to you in hard copy in June) update with m&m thing
Questions for you:
- Pick a diagnostic color-color or color-magnitude plot to make. Does my photometry seem ok?
- Pick at least one color-color or color-magnitude plot to make. Figure out a way to ignore the -9 (no data) flags. Where are the plain stars? Where are the IR excess objects?
- Where are the famous objects in the plot? Where are the new YSO candidates I used the Gutermuth method to find?
- Make a new column in your Excel spreadsheet with some colors. Is there a way you can get Excel to tell you automatically which objects have an IR excess? Can you implement the Gutermuth selection? (You may not be able to do so.)
- Make the plots that go into the Gutermuth selection, including the relevant lines on the plot.
- Of the objects I have that fit the Gutermuth criteria, are any of them false or otherwise bad sources? How can you tell?
- Bonus but very important question: How do you know that some of these sources aren't galaxies? Can you find something that is obviously a galaxy on the images? Can you think of a way using public data that already exist to check on the "galaxy-ness" of some of these objects?
I provided some example plots in ysoplots.ps in each of the sa101 and cg4 directories. In those plots, the YSO candidates that, by my calculation, pass the Gutermuth criteria, are circled. (Check me on this!) However, note that not all of those sources may be real! Overlay them on any of the mosaics using sa101ysocand.reg or cg4ysocand.reg to check on which are real. Moreover, because of the overlap of the mosaics, I may have "rediscovered" the same stars more than once. Overlay the two region files on the same mosaic to check. (Change the symbol color or shape by editing the region file, being careful to save it as a plain text file.)
Big picture goal: Understand what an SED is and why it matters.
More specific shorter term goals: Make at least one SED.
Questions for you:
- Pick some objects to plot up, maybe some of the famous ones from above would be a good place to start, or the ones you flagged above as having an IR excess.
- What do the IR excesses look like in your plots? Do they look like you expected?
I made some SEDs and will email them around to give you an indication of what kinds of numbers you should be getting when you do this calculation for yourself. Many of them do NOT look like I expect. The 24 micron flux may be wrong for some, and the strange shape of some others of them suggest that they are not real and/or background galaxies. Note that I did this just for the objects that I selected via Gutermuth method, and not the set of previously known objects.
This is advanced, and we may not get here.
Add a new column in Excel to calculate the slope between 2 and 8 microns in the log (lambda*F(lambda)) vs log (lambda) parameter space. This task only makes sense for those objects with both K band and IRAC-4 detections. (For very advanced folks: fit the slope to all available points between K and IRAC-4 or MIPS-24. How does this change the classifications?)
- if the slope > 0.3 then the class = I
- if the slope < 0.3 and the slope > -0.3 then the class = 'flat'
- if the slope < -0.3 and the slope > -1.6 then class = II
- if the slope < -1.6 then class = III
These classifications come from Wilking et al. (2001, APJ, 551, 357); yes, they are the real definitions ( read more about the classes here)!
- How many class I, flat, II and III objects do we have?
- Where are the objects with infrared excesses located on the images? Are all the Class Is in similar sorts of locations, but different from the Class IIIs?
Writing it up!
We need to write an AAS abstract and then the poster!
We need to include:
- How the data were taken.
- How the data were reduced.
- What the Spitzer properties are of the famous objects, including how the Spitzer observations confirm/refute/resolve/fit in context with other observations from the literature.
- What the Spitzer properties are of other sources here, including objects you think are new YSOs (or objects you think are not), and why you think that.
- How this region compares to other regions observed with Spitzer.
Take inspiration for other things to include from other Spitzer papers on star-forming regions in the literature.